Traction drive power transmissions which transmit power to a driven part through a traction drive mechanism have attracted attention in the field of automobiles and industrial machinery, and in recent years extensive research and development has been conducted in this area. The traction drive mechanism is a power transmitting mechanism. Unlike conventional drive mechanisms it does not use any gears. This results in a reduction in vibration and noise as well as a smooth speed change in high-speed rotation. An important goal in the automobile industry is improvement in the fuel consumption of automobiles. It has been suggested that if the traction drive is applied to the transmission of automobiles in order to convert the transmission to a continuous variable-speed transmission the fuel consumption can be reduced by at least 20% compared with conventional transmission systems. This is due to the fact that the drive can always be in the optimum fuel consumption region of an engine. Recent studies have been conducted in the areas of development of materials having high fatigue resistance, and in the theoretical analysis of traction mechanisms. Regarding the traction fluid, the correlation of traction coefficients is gradually being understood on a level of the molecular structure of the components. The term "traction coefficient" as used herein is defined as the ratio of the tractional force which is caused by slipping at the contact points between rotators which are in contact with each other in a power transmission of the rolling friction type to the normal load.
The traction fluid is required to be comprised of a lubricating oil having a high traction coefficient. It has been confirmed that a traction fluid possessing a molecular structure having a naphthene ring exhibits a high performance. "Santotrack.RTM." manufactured by the Monsanto Company is widely known as a commercially available traction fluid. Japanese Patent Publication No. 357363/1972 discloses di(cyclohexyl)alkane and dicyclohexane as traction fluids having a naphthene ring. This patent publication discloses that a fluid obtained by incorporating the above-mentioned alkane compound in perhydrogenated (.alpha.-methyl) styrene polymer, hydrindane compound or the like has a high traction coefficient. Japanese Patent Laid-Open No. 191797/1984 discloses a traction fluid containing an ester compound having a naphthene ring. It discloses that an ester obtained by the hydrogenation of the aromatic nucleus of dicyclohexyl cyclohexane-dicarboxylate or dicyclohexyl phthalate is a preferred tractive fluid.
As mentioned above the development of continuous variable-speed transmissions has advanced in recent years. The higher the traction coefficient of the traction fluid the larger the transmission force. This allows a reduction in the size of the device which in turn results in a reduction in emission of polluting exhaust gases. Therefore, there is a strong demand for a fluid having a traction coefficient which is as high as possible. However, the use of a conventional traction fluid which exhibits the highest performance of all the currently commercially available fluids in such a traction drive device provides unsatisfactory performance with respect to the traction coefficient. Such conventional fluids are also expensive. The traction fluid which has been proposed in Japanese Patent Publication No. 357363/1971 contains Santotrack.RTM. or its analogue as a component and, therefore, is also unsatisfactory with respect to performance and cost.
The present inventors have made extensive and intensive studies with a view to developing a traction fluid which not only exhibits a high traction coefficient but is also inexpensive. As a result, the present inventors have discovered that the incorporation of an ester having two cyclohexyl rings or its derivative, or said ester in combination with a branched poly-.alpha.-olefin, can provide an economical, high-performance base oil fluid. The present invention is based on this discovery.